FLEXURE OF THE OCEANIC LITHOSPHERE IN THE VICINITY OF THE MARQUESAS ISLANDS by PAUL

While the visible portion of the Marquesas Islands is much smaller than that of the Hawaiian Islands, modelling of the shipboard gravity collected during the Crossgrain 2 expedition indicates that the rate of magma production by the two hotspots must be similar. The difference in effective elastic plate thickness beneath the two island chains causes the Marquesas to subside to a greater extent, allowing only the late alkalic caps to emerge from the ocean. Since the tholeiitic shields remain immersed, the gravitationally induced stress remains low, and well developed rift zones do not appear. The archipelagic apron surrounding the islands is composed of lava flows from the initial stages of volcanism, and of debris from slumps and catastrophic collapses of the later volcanic edifices. The flexural moat has been completely filled, and the lateral extent of the apron is controlled by flexural uplifting of abyssal hills. Data from the alignment of seamount major axes and dikes observed on the SEABEAM archive plots from the Crossgrain 2 expedition to the Marquesas Islands area are determined to be randomly distributed for seamounts above 500 m in height. For smaller seamounts, the seamount major axes are aligned with the seafloor lineations produced at the mid-ocean ridge. There is no observable correlation between the major axis alignment and dike trends at any seamount height, and no evidence of dike clustering about any direction in the manner observed in the Hawaiian rift zones. Sediments forming the archipelagic apron are observed by single channel seismics to extend out to 250 km from the islands, and to be dammed by uplifted abyssal hills. Ponded sediments can be observed up to 50 km further, burying the low relief between the abyssal hills. The sediments are determined to be at least 1350 m thick near the islands, and to thin to 250 m, the depth of the regional pelagic sediments, at the distal ends of the apron. Velocities for the two units observable in the refraction profiles are estimated at 3.1 km/s and 5.9 km/s. Two of the three units identified in the seismic sections correspond to units seen in similar work in Hawaii, however the completely infilled moat has caused the merging of the offlapping and ponded units observed in Hawaii into a single smooth unit in the Marquesas. The volume of the sediments is estimated to be 118,000 km3, with tens to hundreds of the largest mass wasting events supplying most of the material in the Marquesas as well as in Hawaii. Thick calcareous sedimentary packages fill the inter-island basins, being shed from the steeper slopes